Electrochemical gas sensors are typically used to determine the composition of a gas and may further be used to detect the presence of various elements or compounds in a gas. Electrochemical sensors ordinarily operate at room temperature, provide a signal which varies with concentrations of analyte species, have short response time, and exhibit acceptable sensitivity with high durability. In addition, electrochemical sensors are compact and can be used for continuous monitoring.
Known electrochemical sensors include, using both liquid and solid electrolytic layers, capacitance sensors and surface acoustic wave sensors. However, the sensitivity, or detection capability, of known sensors generally falls in the range of low-ppm or high-ppb.
Other known electrochemical gas sensors typically include gas diffusion electrodes and porous electrolytic films of, for example, Nafion or Teflon. The cornerstone of these sensors generally has been on optimizing the metal/gas/ionic medium interface in order to achieve higher sensitivity. However, the assembly processes for these sensors are manually intensive and are not suited for automated mass production.
U.S. Pat. No. 5,431,806 (“806 patent”), U.S. Pat. No. 5,492,611 (“611 patent”), and U.S. Pat. No. 5,478,460 (“460 patent”) disclose an electrochemical gas sensor having a permeable membrane. These types of sensors typically measure gas by diffusing it through the membrane and dissolving it in an electrolyte on the other side of the membrane. The electrolyte is in contact with electrodes, which in turn typically measure current flow as a function of oxygen concentrations.
U.S. Pat. No. 5,670,031 (“031 patent”) is directed to an electrochemical sensor having a plurality of micro electrodes in series and in close proximity to one another. This sensor typically operates by measuring differences in current flow between one pair of electrodes to the next along a length of a channel in which electrolyte flows. Because minute changes in current flow can be measured, accuracy is generally believed to be improved.
Regardless of the type of electrochemical sensor selected, many typically suffer reduced response time or reduced sensitivity over time. This may occur through simple use of the sensor. Usually after each use, the inside surfaces of electrochemical sensors, particular the sensing electrode, may become coated with impurities. Such impurities include oxidation and may result from the reaction between the gas, electrolytic membrane, electrode surface which is typically metallic, and/or electricity or current. The more the sensor is used and if the sensor is not cleaned, such impurities may lead to corrosion of the electrode surface. Moreover, as impurities, corrosion, or oxidation build up on the surfaces of the sensor's components, sensitivity or sensor response time may be negatively affected.
Typically, ex-situ cleansing of the components' surfaces would usually remove or reduce the above listed impurities, which would often lead to improved sensitivity and response time. However, ex-situ cleansing, which normally involves the removal of the components or dismantling of the sensor, may be time consuming and expensive since these components may require careful handling and an environment free from airborne particles.